In addition to the direct voltage, alternating voltages of mutually different frequencies (ripple voltage) will normally occur over the output of a static converter, for instance a rectifier. The ripple voltage is attenuated, by providing the rectifier with a smoothing filter on the output thereof. FIG. 1 of the accompanying drawings illustrates a rectifier LR provided with one such filter comprising an inductance L and a capacitance C. Great demand is placed on the attenuation of ripple voltage in telephony systems, e.g. on the current supply of telephone exchanges. A permitted level for frequencies around 1 KHz is beneath 1 mV.
The known smoothing filter illustrated in FIG. 1 is a passive filter in which the inductance L consists of chokes and the capacitance C of electrolyte capacitors.
If the fundamental harmonic of the ripple voltage has the frequency fg, the attenuation of the filter is ##EQU1## When X.sub.L &gt;&gt;X.sub.C, as is normally the case, the attenuated can be expressed approximatively as ##EQU2## As a rule, filters used in practice have a powerful attenuation effect on the fundamental harmonic .omega..sub.g of the ripple voltage EQU d=.omega.g.sup.2 LC&gt;&gt;1
and higher frequencies, whereas attenuation of the direct voltage (resistive voltage drop) is negligible.
Filters can comprise one or more LC stages.
Values for a rectifier 48V=400A=are given as an example. L=100 .mu.H C=160 000 .mu.F fundamental frequency of the ripple voltage fg=600 Hz.
The following relationship applies for this frequency ##EQU3## while the attenuation is ##EQU4## The components of rectifier smoothing filters operating at the mains frequency are both large and expensive. In particular, the chokes designed for large direct currents are bulky, heavy and expensive. For example, the choke 400A above weighs 25 kg and costs about 1500 SEK. The rectifier requires two such chokes in order to fulfil the ripple voltage requirement on its output.